A commercially suitable Soft-X-Ray or EUV lithography facility will require an intense soft x-ray/EUV light source that can radiate within a specific wavelength region within the range of approximately 11 to 14 nm in the EUV part of the electromagnetic spectrum. Capillary discharge sources have been proposed that can be used in such a facility. One such embodiment of the proposed capillary discharge source was first described in U.S. Pat. No. 5,499,282 by William T. Silfvast issued on Mar. 12, 1996. That particular proposed source would operate in a lithium vapor electrically excited to within specific ranges of plasma electron temperatures (10-20 eV) and electron densities (10.sup.16 to 10.sup.21 cm.sup.-3) which are required for optimally operating a lithium vapor discharge lamp at 13.5 nm. That same patent also proposed soft-x-ray lamps at wavelength of 7.6, 4.86, and 3.38 nm in beryllium, boron, and carbon plasmas. These wavelengths, however, are not within the range of wavelengths required for EUV lithography. Although that patent described the general features of these lamps, it did not give the specific discharge current operating range that would minimize bore erosion and the emission of debris from the lithium lamp, or the appropriate range of bore sizes for operating such a lamp. That patent did not mention the use of other materials, such as atomic or molecular gases that could be successfully operated in the lamp configurations described in that patent; it naturally follows that neither could it have mentioned what are the preferred operating pressure ranges of those gases that would be suitable for EUV lithography.
Another proposed discharge source for use with EUV lithography was the "differentially pumped capillary" discharge source that was described in U.S. Ser. No. 09/001,696 filed on Dec. 31, 1997, now U.S. Pat. No. 6,031,241, entitled: Capillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and other Related Applications now issued as U.S. Pat. No. 6,031,241, by the same assignee, which is incorporated by reference.
The "differentially pumped capillary" or DPC, allows a lamp that uses a gas as the discharge radiating medium (as opposed to a lamp that uses metal vapors) to operate without a window between the gaseous region and the optics that collect the radiation emitted from the lamp. This is particularly applicable in the 11 nm to 14 nm wavelength region where EUV lithography operates. Because of the very strong absorption of radiation in that wavelength region by all materials, including gases, it is necessary in an EUV lithography system (as well as other applications) to operate the imaging system within a very low pressure environment having a pressure of less than approximately 0.01 Torr. Hence, a lamp would generally need a window to separate the region of the lamp operating in the 0.1 to 50 Torr pressure region from the low pressure region (less than approximately 0.01 Torr) of the imaging system. The DPC allows for the operation of the lamp containing the radiating gas without the need of such a window.
In the operation of the DPC, the gas is inserted into the discharge capillary at the opposite end of the base from that where the radiation flux in the 11 nm to 14 nm radiation is collected. The pressure at the gas inlet end of the capillary would be at the range of from 0.1 to 50 Torr depending upon the particular gas and the desired emission characteristics of the lamp. The gas is pumped through the capillary by having a vacuum pump accessible to the opposite end of the capillary, the end where the radiation flux between 11 nm and 14 nm is collected and is used in the desired optical system such as EUV lithography. As the gas is pumped through the discharge capillary the pressure drops approximately linearly such that it is at the necessary low pressure (less than approximately 0.01 Torr) beyond the capillary. The lamp is operated just like other lamps that have a constant pressure over the length of the capillary bore region by initiating a pulsed discharge current within the capillary in the usual manner. We have observed that there is sufficient pressure within the capillary even at the low pressure side, to produce the desired emission from the lamp and yet the region beyond the lamp has sufficiently low pressure to allow for transmission of the radiation between 11 nm and 14 nm. The capillary itself acts as a retarding system for the gas as it flows through the capillary so that the usage of gas is at a very low rate. The gas can also be recycled back to the high pressure side for reuse.
A problem has been observed with these discharge sources described above. During discharge the interior walls of the capillary erode causing debris to be emitted from the discharge source. The debris can be destructive to the surrounding optics such as concave mirrors immediately adjacent to the capillary bore opening. The emitted debris can form a layer on the mirror, lowering reflectivity and thereby reducing its effectiveness. A layer of just 10 Angstroms (one atomic layer) can stop the mirror from reflecting radiation. Furthermore, large particles in the debris might cause destructive pits on the concave surface of the mirror as well.